The present invention generally relates to computer apparatus and, in a preferred embodiment thereof, more particularly relates to the TEM-shielded support of a circuit board, such as a system motherboard, on a computer chassis.
Printed circuit boards used in various types of computers are typically mounted on a rectangular sheet metal mounting plate that overlies a bottom side wall of a sheet metal chassis structure in turn disposed within a plastic outer housing. During operation of the circuit board electromagnetic interference (EMI) radiation is generated within the board, emanates therefrom, and must be substantially prevented from escaping outwardly through the housing structure. A primary source of EMI radiation generated by a circuit board during its operation is commonly referred to as transverse electromagnetic mode (TEM) radiation and is generated outwardly around the peripheral edge of the circuit board.
TEM radiation is created primarily by the alternating clock current transmitted through the circuit board traces, and the instantaneous current changes in the electronic board components such as chips, and is manifested as a generally wave-shaped mutual inductance disturbance between the spaced apart ground plane and power plane portions of the circuit board disposed within its dielectric substrate member. The TEM wave effect can account for up to about seventy five percent of the total EMI interference generated by the operating circuit board, and creates EMI radiation that is discharged from the edge periphery of the board substrate in a generally parabolic pattern.
In order to provide TEM radiation edge shielding for a circuit board disposed, as described above, on a metal mounting plate secured atop a bottom side wall of a sheet metal chassis it has been a conventional practice to mount the board so that two perpendicular side edges thereof are disposed in a spaced apart, facing relationship with two perpendicular vertical side walls of the chassis, with the other two side edges of the circuit board being in a spaced apart, facing relationship with a perpendicular pair of upturned shielding lip portions of the metal mounting plate. The two perpendicular chassis walls, together with the mounting plate lips, form a TEM radiation "fence" that acts as a barrier to undesirable passage of EMI radiation outwardly through the computer housing.
Under conventional chassis construction practice the rectangular metal circuit board mounting plate is of a one piece construction having a fixed length and a fixed width. Accordingly, as a practical matter, the mounting plate can accommodate only a single size of associated circuit board. For example, a circuit board having either a length or width greater than that of the mounting plate cannot be mounted thereon. Of course, a circuit board smaller than the one for which the mounting plate was designed could be mounted on the support plate. However, at least one TEM-generating edge of the mounted board would be positioned too far away from the TEM "fence" to effectively shield the TEM radiation emanating from that board side edge. Thus, only the designed-for circuit board size can be suitably handled by the fixed size mounting plate.
From a manufacturing standpoint this means that to incorporate a differently sized circuit board (such as a motherboard) in a given computer, the configuration of the board mounting plate, and thus the chassis upon which it is mounted, must be changed. The necessity of reconfiguring the chassis arises from the change in mounting geometry (such as mounting hole locations and the like) occasioned by the different form factor of the revised mounting plate. Having to reconfigure the chassis each time that the motherboard (or other printed circuit board) size needs to be changed, for example to upgrade the computer, represents a considerable manufacturing inefficiency with an attendant increase in the production cost of a given computer line.
Moreover, from the consumer's standpoint this conventional manufacturing limitation can markedly limit his future ability to "upgrade" his computer by replacing the original motherboard with a more powerful but differently sized one at a later date. The consumer must thus often choose between two imperfect options--tolerating a computer that, from a computing power/speed standpoint, is no longer "state of the art", or obtaining this performance increase at the cost of purchasing a complete new computer.
As can be seen from the foregoing, it would be desirable to provide improved TEM-shielding circuit board mounting plate apparatus that eliminates or at least substantially reduces the above-mentioned problems, limitations and disadvantages commonly associated with conventional fixed geometry mounting plate structures as generally described above. It is accordingly an object of the present invention to provide such improved mounting plate apparatus.